This invention relates generally to surface treatment of metals, and more particularly to a method for electrolytically forming composite coating layers of improved properties on the surfaces of metals.
In the field of surface treatment of metals, attempts are being made to impart various novel physical and chemical properties to the surfaces of metals by coprecipitating, in each case, a metallic substance and minute particles for forming a composite coating layer on the metal surface for varying the properties.
Among the requirements for metals, the most essential are those related to the improvements of mechanical and chemical properties such as corrosion resistance, abrasion resistance, and the lubricating nature, as well as aesthetic properties which can be improved by coprecipitating organic substances and the like together with the other substances.
In all of the above mentioned treatments, the agent imparting novel properties to the surface of the metal in each case is not the metal in the composite coating but is the coprecipitate precipitated therewith. The coprecipitated substance generally contains the minute particles which have been suspended in the plating electrolyte and thereafter coprecipitated on the surface of the metal.
The minute particles may comprise molybdenum bisulfide, carbon fluoride, silicon carbide, alumina, or synthetic rubber, depending on the required properties. Regardless of whatever substance is used for the particles, the properties of the particles can affect greatly the properties of the composite coating. The particles may range in size between 0.2 .mu. and 20 .mu., and more preferably between 1 .mu. and 5 .mu..
In the technical field of non-electrolytic nickel plating, which has been studied intensively and utilized widely in industry, the plating of metallic nickel has been carried out in the presence of hypophosphite ions utilized as a reducing agent. However, in non-electrolytic nickel plating, the amount of precipitation of nickel cannot be made proportional to the period of the treatment but is reduced because of the reduction of pH in the nonelectrolytic nickel plating solution. The reason for this is said to be in the generation of phosphorous ions and in the increase of hydrogen ions. Further, the precipitation rate is elevated substantially exponentially with the elevation of the temperature of the electrolytic liquid.
In addition, the characteristic of the plating electrolyte is substantially varied by the increase of ions, and for preventing such variation, it has been necessary to replenish at suitable times the consumable reagents such as Ni.sup.+.sup.+ , (H.sub.2 SO.sub.2).sup.-, OH.sup.-, and a stabilizer. For this reason the conventional non-electrolytic nickel plating process has been accompanied by drawbacks such as short life of the plating electrolyte, elevation of the cost, and difficulty in obtaining uniform quality of the nickel plating.
In consideration of the behavior of phosphorous in non-electrolytic nickel plating wherein a reducing agent of, for instance, sodium hypophosphite is used, it is believed that phosphorous forms a substable amorphous solid solution together with nickel in the precipitated layer, and when the solid solution is thereafter heated, the solution is changed into a stable phase consisting of a Ni-P solid solution and Ni.sub.2 P eutectic crystal. The Ni.sub.2 P changes into Ni.sub.3 P of extremely minute crystalline structure when heated at a temperature of approximately from 380.degree. to 400.degree.C, thereby exhibiting a maximum hardness of the coating layer. Said solid solution, Ni.sub.2 P and Ni.sub.3 P are inclusively called as "nickel phosphide" hereinafter. The creation of the nickel phosphide is found to be contributing to an increase in the hardness of the coating layer. Generally speaking, a coprecipitated layer containing a greater part of phosphorous exhibits a higher hardness and hence a higher corrosion resistance. Ordinarily, a content of phosphorous in the range of from 2 to 4% is considered preferable.
Thus, according to the conventional practice, the properties of the composite layer formed on the surface of a metal have been improved by relying solely on the property of the minute particles coprecipitated with the nickel compound. However, when the content of the minute particles is increased, there arise various difficulties such as a decrease in the adhesiveness of the coating layer to the metal surface, poor uniformity, and poor drawing property of the coating layer. Thus, there is a limitation to increasing the content of the minute particles, whereby the improvement in the physical and chemical properties of the composite coating layer has been restricted.